Separate compilation units and targets for SystemModel

[PhilipFackler]

Description

Move SystemModel implementation to dedicated compilation unit(s).

Closes #408

Proposed changes

Move implementation code for SystemModel to SystemModelImpl.hpp and declares class template instantiations in SystemModel.cpp and SystemModelDependencyTracking.cpp.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• [N/A] There are unit tests for the new code.
• [N/A] The new code is documented.
• The feature branch is rebased with respect to the target branch.
• [N/A] I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

[nkoukpaizan]

Thanks for getting this out of the way!